Synthesis

Why can’t NPs be made via conventional solid state

requires high T for diffusion, overcomes surface energy barriers encouraging sintering and grain growth

top down

milling, laser ablation, sputtering

bottom up

building from atoms/molecules, gives better size/shape control and purity

Solid state (‘Shake and Bake’)

mix powders, calcine (heat to drive off moisture and volatile compounds) at 1200-1500 for days; produces 1-10um grains; useless for NPs, product layer forms between BaO and TiO2 blocking reaction, requires regrinding; energy intesive, slow, coarse, use for bulk ceramics only

Mechanochemical (ball milling)

planetary mill rotates centrifugal force holds material at edge, coriolis force causes balls to smash into material down to nm, local heating to 1000C (few ms), one third of each balls, material, empty space; balls 3 times bigger than largest sample (smaller balls takes longer but gets smaller particles); good for displacement reactions; get contamination from milling media; low crystallinity can require annealing

Explain the difference between thermodynamics dominated and kinetics dominated

a material may be thermodynamically unstable (eg diamond) but the energy barrier to change state is so large that it never happens (becoming graphite); if thermodynamics dominate the phase diagram is obeyed, if kinetics dominate then it may not be obeyed

Hydrothermal (water) / solvothermal (organic solvent)

precursor solution heated (100-400C) in sealed autoclave; decomposition of precursor and precipitation of solid phase; high pressure enables supersaturation for controlled nucleation and growth; produces high crystallinity specific morphologies (spheres, wires, cubes, flowers); can be time consuming to determine optimal parameters due to blackbox nature

Oxalate route

wet chemical precipitation; aqueous solution of desired products mixed with oxalic acid; oxalate chelates metal ions, precipitates insoluble solid of desired stoichiometry; precursor calcined (700-1000C) to decompose to form product;

Pechini route

wet chemical precipitation; precursor mixed with chelating agent (citric acid) to from metal-citrate complexes; alcohol added to trigger polyesterification reaction producing transparent gel; gel dried and heated (pyrolysis) amorphous powder calcined to produce crystalline phasee

Benefits of wet chemical processing

high purity, chemical homogeneity and particle size 25-80nm, requires much lower T than SS reactions

Summary of wet chemical processes

oxalate: precipitation based, metal cations react with oxalate ions forming insoluble metal precursor; simple scalable, good for binary oxides with moderate homogeneity; Pechini: sol-gel related method relying on polymeric resin to trap metal ions in rigid framework; less scalable (gel foaming), better for complex multicomponent oxides; excellent homogeneity

Biotemplating /Saccharose

sugars chelate metal ions via deprotonated hydroxyls (eggbox model), dehydrated causing polymerisation into carbon matrix locking in the metal ions; calcination burns away template leaving porous oxide; single, green, cheap, high surface area; carbon residue risk (foaming or fire) and variable pore size

Sol-gel

wet chemical process for making glassy (vitreous) or ceramic materials; metal alkoxides mixed in correct stoiciometry (sol/colloidal suspension), hydrolysed with water (+acid or base catalyst); polymerisation of hydrolysed species and crosslinking to form transparent (indicates quality) gel; dry to remove volatiles; calcine to crystallise (low temp); excellent molecular homogeneity and purity, good for thin films, low temp, expensive raw materials, long processing times (weeks), not easy to scale

Atomic scale mixing

eliminates long diffusion distances that make SS slow and hot

Nucleation and growth control

diluents, matrix phases, capping agents and low T help nucleate many small particles and prevent too much growth